Bone marrow transplantation is viewed as a potentially invaluable tool for treating different cancers and immune deficiencies. However, little is known about the immunological functions of most bone marrow cells. This is evident by the fact that bone marrow transplantation is not always fully successful because of host versus graft responses (HVGR) and graft versus host disease (GVHD). The role that bone marrow pre-T cells play in these problems is unclear. In vitro models of GVHD and HVGR using bone marrow cells could greatly improve our understanding of these problems. We have developed an in vitro culture system which demonstrates that murine bone marrow cells can generate high levels of cytotoxic effector T-cell activity against alloantigens. This in vitro response was investigated for different T-cell and/or pre-T-cell involvement. Bone marrow cytotoxic responses were dependent upon Lyt 123+ cells. The cytotoxic effector cells generated from alloantigen-stimulated mouse bone marrow cells was characterized as being Lyt 123+ and Lyt 23+. Using mixtures of spleen and bone marrow cells from Thy 1.1 C57BL/6 and Thy 1.2 C57BL/6 mice, it was shown that during alloantigen stimulation of bone marrow cells there is differentiation of Thy 1- cells into Thy 1+ cells. This differentiation was dependent upon allo-antigenic stimulation of mature T\cells present in the bone marrow. However, these newly differentiated Thy 1+ cells did not have cytotoxic activity. Bone marrow cells cultured 5 days in theabsence of alloantigen developed high levels of NK cell-like activity. This activity was associated with cells which differed from splenic NK cells. Bone marrow NK cells were Thy 1+, Asialo GM- and NK 1.1-, whereas splenic NK cells were Thy 1+, Asialo GM1+1 and NK 1.1+. The development of bone marrow NK cells was suppressed when alloantigen was included during the culture. Bone marrow cells stimulated with allogenic spleen cells were shown to produce high levels of interferon alpha/beta (IFN alpha/beta). Production of this IFN alpha/beta was shown to be regulated by alloantigen-stimulated T\cells. IFN alpha/beta was also shown to play a role in differentiation of Thy 1- cells into Thy 1+ cells.